Effects of organic amendment on soil quality as assessed by biological indicators
Sultana, Salma (2011) Effects of organic amendment on soil quality as assessed by biological indicators. [Tesi di dottorato] (Inedito)
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Soil quality decline is one of the most predominant effect deriving from human activities. In particular, intensive agricultural management can affect negatively soils, principally due to rapid depletion of soil organic matter, that affects, in turn, soil physical, chemical and biological properties. The declining trend of soil quality coupled with mismanagement of agricultural production is pose a serious threat to sustainability of intensive agriculture. Sustainable intensive agriculture is essential to sustain food production, provide environmental benefits and harmonious delivery of many ecosystem services including soil and water conservation (Bhardwaj, et al.,2011; Flora, 2010; UNDP, 2010; USDS, 2009; Gowing and Palmer, 2008). On the other hand, there is a growing recognition that soil sustainable management practices, including use of organic amendments, could be a key tool to maintaining soil quality and sustainability in intensive agriculture systems. In view of the fact that, soil organic matter can affect soil quality both directly, by releasing macro and micro elements, and indirectly, by determining changes in soil physical and chemical properties and stimulating, in turn, microbial growth and activity thus preventing depletion in its quality by tillage and harvesting methods. Soil quality is the complex outcome of interactions among physical and chemical characteristics, so its proper assessment requires the determination of a large number of parameter (Bonanomi et al., 2011), but soil quality is also strongly influenced by microbe-mediated processes, and soil functioning can be related to microbial diversity, thus it is likely that microbial community have the potential to serve as an early indication of soil degradation or soil improvement. In fact, it has been showed that biological parameters are sensitive and prompt tools to highlight changes in soil quality. In this study, emphasis has been given to some biological properties of soil, such as microbial biomass, soil respiration rate, soil functional diversity, bacterial community structure and some related indices, as they are highly responsive to stress or disturbance, changes in land-use patterns, tillage practices and management deriving from anthropic activities (Acosta-Martínez et al., 2008). A previous study (Bonanomi et al., 2011, Scotti, 2010) showed that the long term intensive agricultural management under greenhouse negatively affected soil quality and crop yield. In the present study we have tested the hypothesis that the use of organic amendments could restore soil quality also in soils affected for long time by intensive agricultural management and under greenhouse. For this purpose, a field experiment was carried out by providing different types of compost to a soil under intensive agricultural management to assess improvement in its quality by using biological indicators. In particular, the main soil physical and chemical properties (i.e. water content, water holding capacity, pH, organic carbon, total and mineral nitrogen and available phosphorus contents, C/N ratio) and biological characteristics (i.e. microbial biomass carbon, soil potential respiration, microbial quotient, metabolic quotient and coefficient of mineralization) were tested, moreover also soil functional diversity (by substrate induced respiration method) was assayed. As the composting of biomass from agro-industrial waste is indicated as an instrument to return farmland organic substance, a second field experiment was carried out, in an experimental farm managed by La Frontera University (Temuco, Chile), to study effects of addition of pulp sludge from paper industry on soil. In fact, the pulp and paper industry produces large quantities of sludge (by biological wastewater treatment), which is generally rich in organic carbon (such as cellulose, lignin and microorganisms) and inorganic substances (Jordan and Rodriguez, 2004; Zhang et at, 2004; Foley and Cooper band, 2002; Gagnon et at., 2001) and contains low concentration of trace metals and organic pollutants (Gallardo et al., 2010; Gagnon et al., 2001), the application of these composted sludge to agricultural lands, at recommended rates and properly managed, is considered as environmentally friendly disposal (Snyman et al., 1998; Gallardo et al., 2007). In particular, this study aimed to test if use of sludge from pulp and paper industry, as organic amendment, can affect negatively or have lasting and positive effects on chemical and biochemical properties (i.e. organic matter, total N, pH, and P, K, Na, Ca, Mg, Zn and Mn contents FDA-hydrolase and acid phosphatase) and bacterial community structure of the soil (as assessed by PCR-DGGE). Results of the first study, carried out in two farms of the Sele River Plane (southern Italy) under intensive agricultural management and with different geopedologic properties, showed that the continual application of slow-degradable organic fertilizers (compost from municipal wastes mixed with scraps from poplar pruning) affected positively chemical and biochemical/biological properties of the studied soils, but no remarkable effect was found on the functional diversity of the microbial community. In particular, data showed a prompt and lasting increase in organic carbon content after amendment, but the ameliorant effects on all the tested properties of the soils were particularly evident after the second addition. Moreover, the presence of wood scraps in the amendment mixtures favoured a slight increase of C/N ratio, contributing to limit mineralization processes and organic carbon loss from soil in long-term. Although, at tested doses, it was not possible to discriminate among the used amendments, it has to be underlined that the soil with the highest starting values of organic carbon and C/N ratio showed more marked beneficial effects deriving from the amendment, indicating a key role of organic matter content also in promoting soil recovery by sustainable agricultural practices. In conclusion, this study provided useful information for conservation and environmental sustainable management of agricultural soils highlighting that the continual application of organic matter to soil, even in absence of mineral fertilizing, can improve soil chemical and biological properties and, thus, also affect positively soil quality in areas managed for long time by intensive farming. Results of the second study, carried out in an experimental farm of the Universidad de La Frontera (southern Chile) under Lolium perenne cultivation, showed that pulp mill sludge application had generally a positive effect on chemical and biochemical characteristics of soil, although, at the tested doses, no marked or lasting effects were found due to the successive applications. On the contrary, sludge addition did not greatly modify the bacterial community structure, even when high doses of pulp mill sludge were applied. According with previous studies, data showed that pulp mill sludge addiction did not affect negatively soil, considering the heavy metal content in soil, and the beneficial effects deriving from its use were principally due to the increase in micro- and macronutrient contents and microbial activity. Finally, both studies confirmed that the tested biochemical/biological parameters (as microbial biomass carbon, soil potential respiration, microbial indices and enzyme activities) are prompt and sensitive indicators of changes in soil quality due to application of organic amendments. On the other hand, functional or genetic diversity of soil microorganisms was not greatly affected by amendment, probably because of the reduction of competition among microbial population due to the increase in resources, as organic matter and nutrient contents, in amended soils. However, considering the complexity of the problems involved in preventing and mitigating the consequence of the wrong use of the soil, further multidisciplinary studies need to establish the appropriate rate of amendment applications and test the magnitude and stability of beneficial effects deriving from these agricultural practices.
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